The formation of scannable data images on thermal paper is well known. Conventional thermal papers which employ dark inks which reflect and absorb light in the visible spectrum can form images that can be scanned with light in the visible spectrum. Common scannable data images are bar codes which contain strong visible light absorbing pigments or dyes on a white or other light reflecting background and are visible to the naked eye. One disadvantage is that they can be easily duplicated by simply photocopying an original commercial document.
The use of special inks such as fluorescent inks and other optically variable inks to form latent images which are invisible to the naked eye and not reproducible by photocopying is also well known. These latent images are more difficult to reproduce and are typically used as security features. These optically variable inks typically contain a fluorescent compound which responds to infrared or ultraviolet light. Representative disclosures of fluorescing inks include U.S. Pat. No. 4,328,332, issued to Hayes et al. on May 4, 1982, U.S. Pat. No. 4,150,997, issued to Hayes on Apr. 24, 1979 and U.S. Pat. No. 4,153,593.
The use of near infrared fluorescent (NIRF) compounds to form invisible markings is known. For example, the use of near infrared flourescent compounds in security inks for thermal transfer printing has been disclosed in International application WO 97/32733, published Sep. 12, 1997 and Yoshinaga et al., U.S. Pat. No. 5,503,904, also disclose recorded media with invisible identification marks composed of regions of high reflectance and low reflectance in the same near infrared region. In addition, Krutak et al. describe the use of near infrared fluorescent (NIRF) compounds in polyester-based coatings, polyester-amide based coatings and ink compositions which are used for marking articles for identification/authentication purposes, in U.S. Pat. No. 5,292,855, issued Mar. 8, 1994, U.S. Pat. No. 5,423,432, issued Jun. 13, 1995, and U.S. Pat. No. 5,336,714, issued Aug. 9, 1994. Krutak et al. also disclose tagging thermoplastic containers and materials with near infrared flourescent compounds in U.S. Pat. No. 5,461,136, issued Oct. 24, 1995, U.S. Pat. No. 5,397,819, issued Mar. 14, 1995, and U.S. Pat. No. 5,703,229, issued Dec. 30, 1997. Escano et al. disclose inks containing NIRF compounds in U.S. Pat. Nos. 5,614,008 and 5,665,151.
Unlike marks used as security features, a scannable data image defines a region on print media with sufficient precision to provide machine-readable information. To accomplish this, the scannable data image must not only achieve a threshold emission such that it is sensed by a photon detector, it must achieve sufficient contrast with the surface of the print medium such that the location of the boundaries of the image on the print medium can be identified by a logic apparatus via signals from the photon detector. Security features do not require such a level of contrast with the print medium. The security marks need only be sensed for a pass/fail test. While interfering emissions or absorbance from the surface of the print medium with a security mark cannot be ignored, the location of the boundaries of the image is typically irrelevant, such as where the NIRF compound is uniformly (flood) coated on the base sheet or is incorporated in the printed matter.
A print medium commonly used in commercial transactions is thermal paper. Direct thermal paper is a thermosensitive recording material on which print or a design is obtained by the application of heat energy, without an ink ribbon. Thermal paper comprises a base sheet and a coating, and like other coated papers, the coating is applied to give new properties to the base sheet. However, a major distinction in thermal paper from other coated papers is that special color forming chemicals and additives are present in the coatings such that when heat is applied by a thermal head, the color forming chemicals react to develop the desired print or image.
The most common type of thermal coating is the dye-developing type system. The three main color producing components in a dye developing-type thermal paper are a colorless dye (color former), a bisphenol compound or an acidic material (color developer) and a sensitizer. These solid materials are reduced to very small particles by grinding and are incorporated into a coating formulation along with any optional additives such as pigments, binders and lubricants. This coating formulation is then applied to the surface of a base sheet such as paper or other support system and dried. Images are formed on the coated surfaces by the application of heat to melt and interact the three color producing components. The intensity (darkness) of the images formed by the thermal papers depends on the dyes and developers used. Certain dyes and developers can provide images which are scannable with visible light, others do not provide the intensity and thus the requisite contrast for a scannable data image. Where special features are desired for thermal paper, the additives used must not pre-react the reactive components within the thermosensitive coating of the thermal paper to detract from the thermal papers printing performance. Certain chemical factors can adversely affect and degrade the performance of the thermosensitive coating and should be avoided such as some organic solvents (ketones), plasticizers (polyethylene glycol type), amines (ammonia) and certain oils (soy oil).
To protect thermal paper from environmental conditions, and premature coloration from handling, a number of developments have been made. One is to produce a barrier or protective layer on top of the thermal coating (see U.S. Pat. Nos. 4,370,370; 4,388,362; 4,424,245; 4,444,819; 4,507,669; and 4,551,738). Another approach is to encapsulate the reactive components in microcapsules which rupture or become permeable when exposed to heat (see U.S. Pat. No. 4,682,194).
It is desirable to provide a scannable data image on thermal paper which is not visible to the naked eye and can provide secure data and/or function as a security feature.